1. Field of the Invention
This invention relates to a method of fabricating a buried heterostructure semiconductor laser important as a light source for optical-fiber communications.
2. Description of the Prior Art
As structures of semiconductor lasers, widely used are buried heterostructures comprising an active layer which is surroundingly covered with semiconductor materials having a large energy gap and a small refractive index. Semiconductor lasers with this buried heterostructure have superior characteristics such that the they have a low oscillatory threshold current value and a stable oscillation transverse mode, and hence have attracted notice as light sources for optical-fiber communications.
A conventional method for their fabrication is exemplified by the method disclosed in DENSHI JOHO TSUSHIN GAKKAI (Electronic Information Communication Society), Technical Research Report OQE85-8, p.55. This conventional fabrication method is shown in FIGS. 7A to 7E.
As shown in FIG. 7A, first, on an n-InP substrate 101 having a (100) plane as a main plane, an n-InGaAsP light guide layer 501, an InGaAsP active layer 103, a p-InGaAsP buffer layer 502 and a p-InP cladding layer 503 with a thickness of about 1.5 .mu.m are successively epitaxially grown. Thereafter, a mask pattern in a stripe is formed on the p-InP cladding layer in the &lt;011&gt; direction to give an insulating film 504 comprising SiO.sub.2 or Si.sub.3 N.sub.4 (FIG. 7B). Next, after selective etching of the p-InP cladding layer 503 by the use of a solution comprising a mixture of hydrochloric acid and phosphoric acid (FIG. 7C), etching is further carried out using a Br-methanol solution, an etchant for InP and InGaAsP, to form a mesa stripe 505 (FIG. 7D). Finally, after removal of the insulating film 504, a p-InP current blocking layer 108 and an n-InP current blocking layer 109 are formed by a liquid-phase epitaxial growth process in such a way that they are not laid on the cladding layer 503 of the mesa stripe 505. A p-InP burying layer 110 and a p-InGaAsP contact layer 111 are further grown thereon to form a buried heterostructure (FIG. 7E).
However, as a problem in the method of fabricating the above structure, there is the problem that the etching may proceed at a greatly irregular rate to make it difficult to control the width of the stripe of the InGaAsP active layer 103 because the Br (bromine) tends to be evaporated in the etching carried out using the Br-methanol solution to cause the solution to undergo great changes with time.
In addition, the stripe comes to have an inverted mesa form when the etching is carried out using the Br-methanol solution. Here, making thin the p-InP cladding layer 503 is effective from the standpoints of, e.g., decreasing the voltage applied to the above n-InP current blocking layer 109 formed when epitaxial growth for burying is effected, restraining the thyristor operation or output saturation at the time of high-output operation, and also improving temperature characteristics. In usual instances, the mesa stripe 505 is required to have a height large enough for its both sides to be held with the current blocking layers 108 and 109, and hence the etching must be carried out until it has such a height. When, however, the cladding layer 503 is formed in a small thickness of about 1 .mu.m, the InGaAsP active layer 103 comes to be positioned above a constricted part as shown in FIG. 8, so that a (111)A plane 601 tending to form an interfacial level at the sides of the active layer during the crystal growth is exposed. As a result, laser characteristics may become poor as exemplified by an increase in the threshold current values. Our experiments have confirmed this fact.
Besides the Br-methanol solution used here in the etching, a solution comprising a mixture of oxygenated water and hydrochloric acid is also known as an etchant for InP and InGaAsP. When, however, a wafer comprising the p-InP cladding layer 503 on which a mask comprising a stripe-like insulating film has been formed in the &lt;011&gt; direction as in the case of the conventional method is etched with this etchant, the stripe comes to have an inverted mesa form as in the etching with the Br-methanol solution. It is hence impossible to obtain good laser characteristics.
Accordingly, it is necessary to take a method by which the mesa stripe does not come to have an inverted mesa form. As a method that may cause no exposure of (111)A plane 601, there is a method in which a negative resist mask is used. As shown in FIG. 9, since a resist has a poorer adhesion to a semiconductor after than an insulating layer, side etching occurs beneath a resist mask 701 and hence a mesa stripe 700 becomes narrower in the width direction than the resist mask 701. The etching proceeds to generally give a regular mesa form and therefore the (111)A plane 601 tending to form the interfacial level is not exposed. This is a preferable result. Since, however, the resist mask 701 and the p-InP cladding layer 503 are not adhered to each other in a stable state, the form of the mesa stripe 700 formed by etching may change to make the control of stripe width unstable.
According to the conventional epitaxial growth for burying, the uppermost layer of the mesa stripe is the p-InP cladding layer 503, and hence the p-InP cladding layer 503 is directly uncovered when soaked before the current blocking layers are grown. This causes dissociation of P (phosphorus) atoms having a high vapor pressure, and any defects resulting therefrom reach not only the p-InP cladding layer 503 but also the interface between the InGaAsP active layer 103 and the p-InP cladding layer 503 in the case when the p-InP cladding layer 503 has a small thickness, where there is a possibility of lowering the emission efficiency of a laser or adversely affecting its reliability.
As discussed above, there are the problems that, when the etching is carried out using the Br-methanol solution or the solution comprising a mixture of oxygenated water and hydrochloric acid, the stripe comes to have an inverted mesa form, making it difficult to control the stripe width, and, when the p-InP cladding layer is made thin, the (111)A plane 601 is exposed to bring about poor laser characteristics.
In the conventional fabrication method, there is another problem that the p-InP cladding layer can not be made thin so much because the direct exposure of the p-InP cladding layer 503 when the layer is soaked before the current blocking layers are grown by the epitaxial growth for burying, brings about defects in the p-InP cladding layer to cause poor laser characteristics and reliability.